Project Summary/Abstract This proposal is a companion to R35CA210036, which is focused on the role of telomeres in cancer with three areas of particular emphasis: 1. The molecular mechanism underlying the telomere tumor suppressor pathway; 2. The genome instability caused by telomere dysfunction in cancer development; and 3. The opportunity to use telomere biology to gain deeper insights into 53BP1-mediated DSB repair. This R50 requests salary support for Dr. Hiroyuki Takai, who is a highly productive long-term Research Specialist in the Unit Director?s laboratory. Dr. Takai has been a key member of the Unit Director?s laboratory since 2002. His main interest in genome instability in cancer started with his studies of the Chk1 and Chk2 effector kinases of the ATR and ATM DNA damage signaling pathways, respectively. In the de Lange lab, Dr. Takai discovered that the stable expression of the ATR and ATM kinases as well as all other PI3K-related kinases requires Tel2, which he showed acts as a chaperonin (Takai et al. Cell 2007). In a second breakthrough, Dr. Takai was the first to demonstrate that cells detect dysfunctional telomeres as Double-strand Breaks (DSBs), activating the ATM kinase and accumulating DNA damage response factors at chromosome ends. Dr. Takai?s method for detecting telomere damage has become the standard assay the field (Takai et al. Curr. Biol 2003) and his findings explained the role of telomere shortening in cancer. Prior to the activation of telomerase, telomere attrition imposes a proliferative barrier during early cancer development when critically short telomeres activate the DNA damage response and induce senescence or apoptosis. More recently, Dr. Takai has focused on the molecular mechanism of the telomere shortening. Telomere shortening derives in part from the inability of DNA replication to copy the ends of linear DNAs. However, the greatest factor in telomere attrition is the 5? resection of telomere ends after their replication, a process needed to generate the protective 3? telomeric overhangs. After this process, excessive 5? end resection at telomeres is counteracted by Pola/Primase mediated fill-in synthesis (Wu, Takai, and de Lange, Cell 2012). Dr. Takai showed that Pola/primase is recruited to telomeres through the interaction of its accessory factor, CST, with the telomeric shelterin complex. Dr. Takai next showed that in absence of CST/Pola/primase, unmitigated resection leads to stochastic shortening of the 5? ended strand and telomere loss (Takai et al. GenesDev 2016). Finally, Dr. Takai was instrumental in a study from the Unit Director?s laboratory establishing an analogous role for CST/Pola/primase in the repair of DSBs (Zirman et al. Nature 2018). A long-term member of the Unit Director?s group and a highly-skilled, rigorous, creative, and collaborative scientist, Dr. Takai is a pivotal contributor to the R35 research program. Dr. Takai is highly committed to the Unit Director?s research on the role of telomeres in cancer and his career goal is to continue to excel and support the success of the R35 through innovative and path-breaking research.